Springboot源码分析之jar探秘

摘要：

• 利用IDEA等工具打包会出现springboot-0.0.1-SNAPSHOT.jar,springboot-0.0.1-SNAPSHOT.jar.original,前面说过它们之间的关系了，接下来我们就一探究竟，它们之间到底有什么联系。

文件对比：

• 进入target目录,unzip springboot-0.0.1-SNAPSHOT.jar -d jar命令将springboot-0.0.1-SNAPSHOT.jar解压到jar目录
  
• 进入target目录,unzip springboot-0.0.1-SNAPSHOT.jar.original -d original命令将springboot-0.0.1-SNAPSHOT.jar.original解压到original目录
  

前面文章分析过springboot-0.0.1-SNAPSHOT.jar.original不能执行，将它进行repackage后生成springboot-0.0.1-SNAPSHOT.jar就成了我们的可执行fat jar,对比上面文件会发现可执行 fat jar和original jar目录不一样，最关键的地方是多了org.springframework.boot.loader这个包,这个就是我们平时java -jar springboot-0.0.1-SNAPSHOT.jar命令启动的奥妙所在。MANIFEST.MF文件里面的内容包含了很多关键的信息

Manifest-Version: 1.0
Start-Class: com.github.dqqzj.springboot.SpringbootApplication
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Build-Jdk-Spec: 1.8
Spring-Boot-Version: 2.1.6.RELEASE
Created-By: Maven Archiver 3.4.0
Main-Class: org.springframework.boot.loader.JarLauncher

相信不用多说大家都能明白Main-Class: org.springframework.boot.loader.JarLauncher是我们 java -jar命令启动的入口，后续会进行分析，Start-Class: com.github.dqqzj.springboot.SpringbootApplication才是我们程序的入口主函数。

Springboot jar启动源码分析

public class JarLauncher extends ExecutableArchiveLauncher {
    static final String BOOT_INF_CLASSES = "BOOT-INF/classes/";
    static final String BOOT_INF_LIB = "BOOT-INF/lib/";

    public JarLauncher() {
    }

    protected JarLauncher(Archive archive) {
        super(archive);
    }
   /**
    * 判断是否归档文件还是文件系统的目录 可以猜想基于文件系统一样是可以启动的
    */
    protected boolean isNestedArchive(Entry entry) {
        return entry.isDirectory() ? entry.getName().equals("BOOT-INF/classes/") : entry.getName().startsWith("BOOT-INF/lib/");
    }

    public static void main(String[] args) throws Exception {
    /**
     * 进入父类初始化构造器ExecutableArchiveLauncher
     * launch方法交给Launcher执行
     */
        (new JarLauncher()).launch(args);
    }
}

public abstract class ExecutableArchiveLauncher extends Launcher {
    private final Archive archive;

    public ExecutableArchiveLauncher() {
        try {
    /**
     * 使用父类Launcher加载资源，包括BOOT-INF的classes和lib下面的所有归档文件
     */
            this.archive = this.createArchive();
        } catch (Exception var2) {
            throw new IllegalStateException(var2);
        }
    }

    protected ExecutableArchiveLauncher(Archive archive) {
        this.archive = archive;
    }

    protected final Archive getArchive() {
        return this.archive;
    }
    /**
     * 从归档文件中获取我们的应用程序主函数
     */
    protected String getMainClass() throws Exception {
        Manifest manifest = this.archive.getManifest();
        String mainClass = null;
        if (manifest != null) {
            mainClass = manifest.getMainAttributes().getValue("Start-Class");
        }

        if (mainClass == null) {
            throw new IllegalStateException("No 'Start-Class' manifest entry specified in " + this);
        } else {
            return mainClass;
        }
    }

    protected List<Archive> getClassPathArchives() throws Exception {
        List<Archive> archives = new ArrayList(this.archive.getNestedArchives(this::isNestedArchive));
        this.postProcessClassPathArchives(archives);
        return archives;
    }

    protected abstract boolean isNestedArchive(Entry entry);

    protected void postProcessClassPathArchives(List<Archive> archives) throws Exception {
    }
}

public abstract class Launcher {
    public Launcher() {
    }

    protected void launch(String[] args) throws Exception {
      /**
       *注册协议处理器，由于Springboot是 jar in jar 所以要重写jar协议才能读取归档文件
       */
        JarFile.registerUrlProtocolHandler();
        ClassLoader classLoader = this.createClassLoader(this.getClassPathArchives());
      /**
       * this.getMainClass()交给子类ExecutableArchiveLauncher实现
       */
        this.launch(args, this.getMainClass(), classLoader);
    }

    protected ClassLoader createClassLoader(List<Archive> archives) throws Exception {
        List<URL> urls = new ArrayList(archives.size());
        Iterator var3 = archives.iterator();

        while(var3.hasNext()) {
            Archive archive = (Archive)var3.next();
            urls.add(archive.getUrl());
        }

        return this.createClassLoader((URL[])urls.toArray(new URL[0]));
    }
    /**
     * 该类加载器是fat jar的关键的一处，因为传统的类加载器无法读取jar in jar模型，所以springboot进行了自己实现
     */
    protected ClassLoader createClassLoader(URL[] urls) throws Exception {
        return new LaunchedURLClassLoader(urls, this.getClass().getClassLoader());
    }
   
    protected void launch(String[] args, String mainClass, ClassLoader classLoader) throws Exception {
        Thread.currentThread().setContextClassLoader(classLoader);
        this.createMainMethodRunner(mainClass, args, classLoader).run();
    }
    /**
     * 创建应用程序主函数运行器
     */
    protected MainMethodRunner createMainMethodRunner(String mainClass, String[] args, ClassLoader classLoader) {
        return new MainMethodRunner(mainClass, args);
    }

    protected abstract String getMainClass() throws Exception;

    protected abstract List<Archive> getClassPathArchives() throws Exception;
   /**
          * 得到我们的启动jar的归档文件
            */
    protected final Archive createArchive() throws Exception {
        ProtectionDomain protectionDomain = this.getClass().getProtectionDomain();
        CodeSource codeSource = protectionDomain.getCodeSource();
        URI location = codeSource != null ? codeSource.getLocation().toURI() : null;
        String path = location != null ? location.getSchemeSpecificPart() : null;
        if (path == null) {
            throw new IllegalStateException("Unable to determine code source archive");
        } else {
            File root = new File(path);
            if (!root.exists()) {
                throw new IllegalStateException("Unable to determine code source archive from " + root);
            } else {
                return (Archive)(root.isDirectory() ? new ExplodedArchive(root) : new JarFileArchive(root));
            }
        }
    }
}

public class MainMethodRunner {
    private final String mainClassName;
    private final String[] args;
   
    public MainMethodRunner(String mainClass, String[] args) {
        this.mainClassName = mainClass;
        this.args = args != null ? (String[])args.clone() : null;
    }
    /**
     * 最终执行的方法，可以发现是利用的反射调用的我们应用程序的主函数
     */
    public void run() throws Exception {
        Class<?> mainClass = Thread.currentThread().getContextClassLoader().loadClass(this.mainClassName);
        Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
        mainMethod.invoke((Object)null, this.args);
    }
}

小结：

内容太多了，未涉及归档文件，协议处理器，打包war同样的可以用命令启动等，感兴趣的读者请亲自去调试一番，添加依赖

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-loader</artifactId>
        </dependency>

IDEA进行启动类的配置